/Engine/Plugins/Importers/USDImporter/Source/USDClasses/Public/USDAssetCache2.h
Describes the different "types of storage" we allow on the asset cache. Note: Order is very important here. The rule of thumb is that it should always be OK for an asset in a storage of lower value to have dependencies on assets in storage of higher value (e.g. it's OK for a material in referenced storage to depend on textures in persistent storage, butnotthe other way around!)
enum ECacheStorageType { None = 0, Unreferenced, Referenced, Persistent, }